Partly it's because Reddit is in an antagonistic relationship with its users. People want a good message board. A good message board doesn't make for very good advertisements. What do you advertise on /r/ChangeMyView ?

Partly it's because Reddit has squandered its users' goodwill. I'd be willing to pay for Reddit if it was clear Reddit was going to work in my favor. Since right now it's doing its best to run off the mobile app I'm using, why on earth would I do that?